1. Technical Field
The present invention relates to a liquid ejecting apparatus typified by a fax machine and a printer.
2. Related Art
An ink jet printer is described below as an example of a liquid ejecting apparatus. The ink jet printer includes a support member (also called a platen) at a position facing an ink jet recording head. The support member supports a recording sheet so as to define a distance between the ink jet recording head and the recording sheet.
In recent ink jet printers, ink droplets have been reduced in size to further improve recording quality. For example, an ink droplet has been reduced in size to be about a few pico liters. Because of this reduction in size, the ink droplet has an extremely small mass. Some of such minute ink droplets do not land on the recording sheet and float as a mist after being ejected from the ink jet recording head toward the recording sheet, thereby causing various problems. Additionally, the floating mist phenomenon becomes much more pronounced in what is called borderless recording. Because, borderless recording is carried out on a recording sheet in such a manner that no margins remain at the four sides of the recording sheet, ink droplets are also ejected onto a region beyond the edges of the recording sheet.
In order to cope with such problems, a technique has been proposed in which a potential difference is provided among an ink jet recording head, a recording sheet, and a support member so as to generate an electric field, which exerts a Coulomb force on ink droplets to attract the ink droplets to the recording sheet. The technique is shown in JP-A-2007-118321 and JP-A-2007-118318, which are examples of related art.
1. Problems Associated with High-Speed Transportation of Recording Sheets
Recently, an ink jet printer has been proposed that can execute recording with an extremely high throughput by using a fixed ink jet recording head, i.e., what is called a line head, in which scanning (movement) is not carried out. In such an ink jet printer, the recording sheets are transported at an extremely high speed in a sheet transportation path inside thereof.
However, the following problems arise in association with such high-speed transportation of recording sheets. When the potential among the recording sheet, the support member (the platen), and the ink jet recording head (these three elements are collectively referred to as “recording section constituent elements” hereinafter) is not controlled, an electric field is generated among the recording section constituent elements. The electric field causes paper powder that is produced in cutting and sticks to the edges of the recording sheets to fly off toward the ink jet recording head and stick onto the ink jet recording head. Particularly, when the recording sheets are transported at a high speed, vibration and shock during the sheet transportation become large. As a result, flying off of the paper powder becomes much more pronounced.
Additionally, with increasing friction between the recording sheets housed in a sheet cassette, and sliding contact and contact between the constituent elements (e.g., an edge guide and a transportation roller) in the sheet transportation path and the recording sheets, frictional electrification and peeling electrification become much more pronounced. Consequently, the recording section constituent elements are more pronouncedly charged. An electric field generated among the recording section constituent elements becomes strong and the paper powder is also further charged. This increases a Coulomb force acting on the paper powder, so that the sticking of the paper powder to the ink jet recording head becomes much more pronounced.
Further, even if the paper powder is not charged, the paper powder flying off in the electric field is attracted to a side adjacent to the ink jet recording head, because electric charges are biased in the paper powder by dielectric polarization or static induction.
FIG. 5 is an explanatory view of the problem. In FIG. 5, an ink jet recording head 160, a nozzle plate 160a, a support member (a platen) 170, a rib 170a formed on the support member 170, a recording sheet P having a sheet edge section Pe, and paper powder d are shown. Circled plus signs and circled minus signs show the respective charged polarities.
The electric charges of the recording sheet P are eliminated by an anti-static brush or the like. Thus, the paper powder d stuck to the recording sheet P is not charged. However, when the nozzle plate 160a is positively charged and the support member 170 is negatively charged (as an example), negative electrical charges appear in the paper powder d at a side adjacent to the nozzle plate 160a while positive electrical charges appear at a side adjacent to the support member 170 due to dielectric polarization or static induction in the case shown in an enlarged view of a particle of the paper powder d. As the result, the paper powder d is attracted to both the nozzle plate 160a and the support member 170. Here, dielectric polarization occurs when paper powder d has characteristics of a dielectric material while static induction occurs when paper power d has characteristics of a conductor.
The paper powder stuck to the ink jet recording head directly blocks nozzle apertures or is moved to the nozzle apertures when a nozzle surface is cleaned (in wiping), thereby causing missing dots.
Besides the case where the paper powder physically blocks the nozzle apertures, a case may arise where a loading material, such as calcium carbonate, included in the paper powder reacts with water in ink to increase the viscosity of the loading material, thereby hindering meniscuses of the nozzle apertures from vibrating and ink droplets from being ejected as well. Consequently, preventing the paper powder from sticking to the ink jet recording head is very important to achieve appropriate recording quality in the ink jet printer.
2. Problems in Related Art
In JP-A-2007-118321 and JP-A-2007-118318, the technique is proposed in which a potential difference is provided among the ink jet recording head, the recording sheet, and the support member (the recording section constituent elements) so as to generate an electric field, which exerts a Coulomb force on ink droplets to attract the ink droplets to the recording sheet, as described above. With reference to the technique, it may be considered that the sticking of the paper powder to the ink jet recording head can be prevented by controlling an electric field so as to attract the paper powder to the recording sheet side if the paper powder is treated in the same way as the ink droplets.
However, the loading material and cellulose fibers that are included in the paper powder are easily charged with both a positive polarity and a negative polarity from the point view of triboelectric series. Accordingly, an attempt to prevent the paper powder from flying off toward the ink jet recording head by forming an electric field in a specific direction among the recording section constituent elements will fail, because this attempt cannot prevent the paper powder charged with the opposite polarity to that of the recording section constituent element from flying off toward the ink jet recording head.
JP-A-2003-165230 discloses a recording apparatus having an air duct that is provided around a nozzle plate and prevents paper powder, dust and the like from sticking around a nozzle section of an ink jet recording head as one of the objectives thereof. In the apparatus, humidified air is ejected from the air duct in a recording state and a recording waiting state. The structure, however, causes the apparatus to become larger and to increase costs due to the increased complexity of the structure. Additionally, airflow may promote sticking of the paper powder to the recording head.
JP-A-2008-213255 discloses a technique in which a paper powder collection member conductivity collects paper powder. This technique also has the problem of the opposite polarity described as above, so that the paper powder cannot be consistently and efficiently collected. Additionally, a problem arises regarding treatment (removal) of the paper powder deposited on the paper powder collection member. Particularly, in a state where much paper powder has been deposited, the paper powder may fly off into surrounding spaces due to a small vibration or shock. The technique, thus, has a problem of how to maintain the performance in the long term.